Electronic systems often have many integrated circuits (IC's) mounted onto printed-circuit board (PCB's) that connect to other PCB's. The IC's typically have metal contact pads, pins, or balls that are soldered to metal pads on the PCB. The PCB may also have pads or contacts that mate with a connector or socket mounted to another PCB or to a backplane in a chassis. This type of interconnect features contacts that make a physical and electrical connection.
However, at very high data rates the performance of the physical interconnect is affected by the materials surrounding the electrical contact as well as the geometries of the contacts themselves, which can both limit the usable frequency band for signals across the connector. Such a physical interconnect often behaves like a low pass filter, with some resonant frequencies resulting from the contact geometries that approach the dimensions of the wavelength of the signal.
Interconnects that do not have a physical, direct contact are also used. Such contactless interconnects include radio-frequency (RF) signals that are modulated and transmitted by an antenna, to be received by another antenna. However, RF modulation, transmission, and reception is quite complex and costly. Wideband couplers are also used with RF equipment to split signals of different frequencies from different sources. However, wideband couplers tend to be on a single substrate, which prevents removal and replacement of circuit boards in a system.
FIG. 1 shows a capacitive coupler interconnect. A signal on chip 18 is driven by buffer 10 onto pad 14, which does not make physical contact with pad 16 on chip 20. Instead, pads 14, 16 are separated by a thin air gap 22. If air gap 22 is sufficiently thin, relative to the areas of pads 14, 16, air gap 22 acts as a dielectric in a capacitor, with pads 14, 16 acting as the plates of the capacitor.
The signal driven onto pad 14 by buffer 10 is capacitively coupled across air gap 22 to create a signal on pad 16. The signal on pad 16 can be amplified by receiver 12 to be used by other circuitry on chip 20.
The signal coupled to pad 16 has the same polarity as the signal on pad 14, but its amplitude is smaller due to parasitic losses. Other parasitic capacitances on the node between pad 16 and amplifier 12 cause charge sharing that reduces the amplitude of the coupled signal. Differential signaling may be used to increase the signal-to-noise ratio and thus compensate for the signal attenuation.
The performance of such capacitive couplers depends on the thickness and type of the dielectric between pads 14, 16 which includes air gap 22 and perhaps some dielectric layers above pads 14, 16, or even a gel or other paste placed in air gap 22. Variations in the dielectric thickness can significantly alter the interconnect performance. The thickness of air gap 22 must be extremely small to produce a significant capacitance. Achieving a small and controlled thickness is challenging, especially for interconnect between PCB's.
What is desired is an improved contactless interconnect.